Ion Beam Stabilization of FePt Nanoparticle Arrays for Magnetic Storage Media
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Ion Beam Stabilization of FePt Nanoparticle Arrays for Magnetic Storage Media J.E.E. Baglin1,2, Shouheng Sun3, A.J. Kellock1, T. Thomson1,4, M.F. Toney1,5, B.D. Terris1,4 and C.B. Murray3 1
IBM Almaden Research Center, San Jose, CA 95120; 2Materials Research Institute, Northwestern University, Evanston, IL 60208-3116; 3IBM T.J. Watson Research Center, Yorktown Heights, NY 10598; 4Hitachi San Jose Research Center, San Jose, CA 95120. 5 Stanford Synchrotron Radiation Laboratory, SLAC, Menlo Park, CA 94025. ABSTRACT We describe the use of ion beam induced crosslinking to harden the organic matrix material of self-assembled arrays of monodisperse (4 nm) FePt nanoparticles, providing diamondlike carbon barriers to inhibit agglomeration of the nanoparticles under heat treatment. Such stabilization is necessary for the particles to survive the >500°C annealing required for growth of the fct L10 phase of FePt, whose magnetic anisotropy is necessary for application of such arrays for high density recording. Selective area irradiation of continuous nanoparticle coatings, using ion beams patterned over a full disk by stencil mask or with ion projection optics, followed by dissolution of the unexposed coating, is proposed as a means of fabricating extended bit patterns consisting of isolated “islands” of FePt nanoparticles, with characteristic dimensions of tens of nanometers. INTRODUCTION The magnetic disk drive industry has recently seen demonstrations of storage density exceeding 100 Gb/in2, and it seeks to continue increasing that density without loss of device reliability or appreciable increase of manufacturing complexity and cost. This suggests the need for pre-patterned magnetic media, with nanometer-scale features. The attractive concept of selfassembled FePt nanoparticle arrays, as described by Sun et al. [1,2,3], would offer ordered patterns of magnetically isolated 4nm particles, provided that the assembly could retain its integrity during the >500°C heat treatment required for formation of the chemically ordered fct ferromagnetic phase of FePt, whose substantial magnetic anisotropy could be viable for magnetic recording. However, at such temperatures, the organic matrix of the particles is not stable, and agglomeration of the particles may take place, degrading the initially tight distribution of particle sizes, and the integrity of the particle array. In this paper, we report some preliminary success of ion beam irradiation of the organic-embedded layer prior to heating, as a means of inhibiting this agglomeration and generating a stable, hard medium. Since we have currently no viable technology for reading or writing single nanoparticles as independent bits, we also propose a way in which the ion-stabilized nanoparticle medium would lend itself to robust, full-disk patterning with bits consisting of isolated islands of FePt nanoparticles, having practically addressable dimensions up to tens of nanometers. Such patterning would represent a challenge for standard optical lithography, with its interferencel
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